The present invention relates to an application device, and more particularly to an improved application device for performing continuous application of a coating liquid for forming a magnetic recording layer on a flexible carrier.
A magnetic recording medium of the application type has been widely used in various fields of industry. To manufacture a magnetic recording medium of the application type, a coating liquid suitable for the application at hand is applied in a desired thickness to the surface of a continuously moving flexible nonmagnetic band-like carrier (hereinafter referred to as web), which is then dried. Such a recording medium manufacturing technique is very much different from a process of manufacturing a magnetic recording medium of the nonapplication type, which is manufactured through ion plating, sputtering, vacuum evaporative deposition or the like.
Most magnetic recording media which have been recently manufactured and sold for audio, video and data processing applications are of the application type, namely, manufactured using an application process such as a gravure process, a reverse roller process, a doctor blade process or an extrusion process. However, with these conventional application processes it is difficult to greatly enhance the productivity of the medium because there is a problem in the accuracy of application in such processes. For example, it is difficult to increase the speed of application since the thickness of the web is generally very small and the applied liquid for forming the ferromagnetic layer is thixotropic, which greatly affects the electromagnetic conversion property of the magnetic recording medium. As for an application process in which there are a relatively large number of places where stagnation can occur in the passage for the liquid for forming the ferromagnetic layer, an abnormal rise in the viscosity of the liquid or the cohesion thereof is likely to occur due to the thixotropy of the liquid, resulting in a large variation of the electromagnetic conversion property of the ferromagnetic layer. If the quantity of liquid discharged from the slot of the extruder of a conventional extrusion-type application device is decreased, the liquid is likely to cohere in a plane opposite the liquid supply nozzle through which the liquid is supplied into the liquid reservoir of the extruder. The cohesion of the liquid results in the formation of a large number of longitudinal streaks in the layer of the liquid applied to the web. On the other hand, if the quantity of the applied liquid discharged from the slot of the extruder is increased, the number of such longitudinal streaks decreases, but a color nonuniformity (a thickness nonuniformity) in the layer of the liquid applied to the web tends to occur and spread over the entire width of the layer. What causes the longitudinal streaks and the color nonuniformity has been found to be that speed of the flow of the liquid in the longitudinal direction of the extruder greatly affects the thixotropy of the liquid. More specifically, it has been determined that the viscosity of the liquid near the liquid supply nozzle, where the speed of the liquid is relatively high, is likely to drop due to a shearing action so as to cause the color nonuniformity. The flow velocity of the portion of the liquid opposite the supply nozzle drops to nearly zero, making the liquid likely to cohere and cause longitudinal streaking.
As a result of this determination, an improved application method was proposed, as disclosed in the Japanese Patent Application No. 53674/85. In accordance with this method, an applied liquid is continuously discharged from the outlet portion of a slot facing the surface of a continuously moving web so as to form a thin film of the applied liquid on the surface of the web. The method is characterized in that the quantity of liquid supplied to the liquid reservoir of the extruder which communicates with the slot is made larger than that of the liquid actually applied to the web, and a portion of the liquid in the reservoir is caused to flow out not through the slot but from a portion of the reservoir located farthest or nearly farthest from the liquid supply nozzle for the reservoir along the width of the web.
Another application method in which a liquid portion supplied separately from liquid supplied to the reservoir is forcibly extracted has been disclosed in Japanese Patent Application (OPI) No. 236968/89 (the term "OPI" as used herein means an "unexamined published application"). As for these methods, the numbers of longitudinal streaks and color nonuniformities have been much reduced in comparison with the prior practice so as to improve the electromagnetic conversion property of ferromagnetic layers formed using such methods.
An application device including an extrusion-type application head designed in accordance with the formula indicated below has been disclosed in Japanese Patent Application (OPI) No. 180266/89: ##EQU2##
In the formula, Q.sub.1 -Q.sub.2, t, S, L and D denote the applied quantity of a liquid, the width of a slot, the length of the slot, the depth of the slot, and the diameter of a liquid reservoir, respectively. However, according to the formula, it is necessary in reality to make the length S of the slot very large or make the width t of the slot very small. If the length of the slot is made very large, the size of the discharge portion of the application device is very large. If the width of the slot is made small, foreign matter is very likely to be trapped in the slot to cause a very large number of longitudinal streaks in the film of applied liquid. Therefore, the device is not practical is use. Although the formula is based on the supposition that the applied liquid forms a Poiseuille flow in the reservoir, the actual applied liquid is a very non-Newtonian one whose apparent viscosity affects the physical properties of the liquid and depends on the flow of the liquid itself. Since the apparent viscosity of the liquid changes in the liquid reservoir and the slot, the thickness of the film of the applied liquid can be made uniform by selecting the dimensions of portions of the device.
The density of recording in magnetic recording media has become higher and higher in recent years. As a result, the media have been required to be higher in quality than prior ones. Therefore, an extrusion-type application device by which a very high effect is produced in consideration of the thixotropy of the applied liquid at the time of the application thereof has been desired.